1. Field of the Invention
The invention relates to the chloromethylation of aromatic compounds and in particular to the chloromethylation of sulfur containing compounds, such as thiophene, to produce 2-chloromethylthiophene used as an intermediate in the production of thiophene 2-acetonitrile.
2. Related Art
Chloromethylation, i.e., the replacement of a hydrogen atom by a chloromethyl group is a known reaction. A commonly found method involves the use of formaldehyde and hydrogen chloride in the presence of a converting agent such as zinc chloride. Various methods of conducting the chloroalkylation of sulfur-containing hydrocarbons have been disclosed, see U.S. Pat. Nos. 2,527,679; 2,541,408 and 2,951,000.
In particular 2-chloromethylthiophene (CMT) has been prepared by the addition of a concentrated aqueous formaldehyde solution to a mixture of thiophene and concentrated hydrochloric acid and passing in a rapid stream of hydrogen chloride. This batch process is said to require strict constraints on hydrogen chloride gassing rates and heat removal to maximize yields. With no equipment constraints, however, the hydrogen chloride addition rate remains limited on the low side by a 6 hour maximum reaction time in a temperature range of -20.degree. C. to +10.degree. C. Long reaction times cause the system to foul. The fouling is caused by the formation of polymeric or oligomeric material of high viscosity such as thiophene formaldehyde oligomers. When this happens the reaction mixture is highly emulsified and the product, 2-chloromethylthiophene, will not separate out even with a deemulsification agent. High hydrogen chloride addition rates were also thought to be limited by formation of high temperature zones, which also favored oligomeric by-product formation and caused the system to foul. The heat to be removed is generally a function of the gassing rate and the desired temperature path over the reaction time. A normal plant reactor system consisting of a vessel, pump and cooler further limit the hydrogen chloride gassing range by its design characteristics.
An object of the invention is to provide a means of increasing the yield of the desired chloromethylated product with time and to minimize the formation of thiophene-formaldehyde oligomers which contain the poly(thienyl methane), or ##STR1## structure.